Semiconductor device having a composite film as a passivating film

ABSTRACT

A semiconductor device having a composite film as a passivating film formed on the surface of the device in the order of films of silicon oxide formed by thermal oxidation, phosphorous glass, silicon oxide formed by chemical vapor deposition and silicon nitride, whereby cracks, which occurs in the film when the phosphorous glass film is in contact with the silicon nitride film, can be eliminated.

D United States Patent 1 [111 3,745,428 Misawa et al. 1 July 10, 1973 [54] SEMICONDUCTOR DEVICE HAVING A 3,507,716 4/1970 Nishida et al 317/235 COMPOSITE FILM AS A PASSIVATING 3,462,657 8/1969 Brown 317/235 FILM 3,560,810 2/1971 Balk et al 317/235 3,566,517 3/1971 Brown et a1. 317/235 [75] Inventors: Yutaka Misawa; Takuzo Ogawa; 3,575,743 4/1971 Chiovarou etal. 317/235 Hideyuki Yagi, all of l-litachi-shi, Japan Primary Examiner-Jerry D. Craig [73] Asslgneez Hltachl, Ltd., Tokyo, Japan Attorney craig and Amend [22] Filed: Feb. 1, 1971 [21] Appl. No.: 111,267

[57] ABSTRACT [30] Foreign Application Priority Data A semiconductor device having a composite film as a Jan. 30, Japan passiyating formed on the urface of the device in i the order of films of silicon oxide formed by thermal U.S. CL R, oxidation phosphorous glass ilicon oxide formed [51] Int. Cl. H011 7/00 h i l vapor deposition and silicon nitride, whereby [5 8] Field of Search 317/275 cracks, hi h occurs i the film when the phosphorous glass film is in contact with the silicon nitride film, can [56] References Cited be eliminated UNITED STATES PATENTS 9/1967 Miller et a]. 317/235 3 Claims, 11 Drawing Figures III/l wean? THICKNESS OF 'Si N |O 5) PATENIEU J 3. 745 .428

SHE" 1 0F 5 400 800 |200 [04X THICKNESS OF Si N (A) FIG. 2

l iZCRACK OCCURRNCE 1: NO CRACK I000 2000 o 3000 THICKNESS OF P O -SiO (A) INVENTORS nrrdRNEYS PAIENIED JUL i o um SHEEIZBFS iICRACK OCCURRENCE IINO CRACK FIG. 3

m. 8 6 4 2 xwzmw no mmmzxe TI 850 900 V TEMPERATURE OF' DEPOSITION ("Cl m m0 wwmZxuEF 0 1666 z o b l dg):

M THICKNESS OF cvo saozm INVENTORS YUTRKR P'HsQWQ, TRKUZO OGHWR PAIENTEDJUUOIQYS 3,745,428

suwsnfs FIG. 7C!

FIG. Th3

QTTORNEYE The present invention relates to an improved technique for passivating a semiconductor device, and more particularly to a semiconductor device having a composite film free from cracks occurring therein.

A great number of techniques for passivating semiconductor devices have been suggested heretofore in order to protect the devices from undesirable irreversible changes of electric parameters thereof. Since the surface of the semiconductor device is active and tends to be influenced by environment, it must be protected by a suitable passivating film. i

The kind of film to be employed is chosen from the view point of the passivating effect thereof. In a type of a semiconductor device in which at least one edge of a PN junction extends to a plane surface of the semiconductor body, the free surface including the surface of the edge is covered with a silicon dioxide film which is formed by thermal oxidation on the surface of the body. The silicon dioxide film thus formed will be referred to hereinafter as SiO (T.O.) film. The SiO (T.O.) film has been used not only as a passivating film but also as a masking film for doping impurity into the body. The SiO (T.O.) film has been recognized as one of the most suitable passivating films because it produces only small surface effects on the PN junction. Especially, it generates only a small charge density N in the surface of the body. From the above-mentioned points of view, the SiO (T.O.) film has been used widely as a passivating film.

In practice, however, the SiO, (T.O.) film should be covered with a phosphorous glass comprising a glassy mixture of phosphorus pentoxide P and silicon dioxide SiO since the SiO (T.O.) film has a considerably poor alkali-ion resistance. In other words, alkali substances, such as, sodium ions, lithium ions or the like, which tend to be drawn into the SiO, (T.O.) film during the processes of impurity doping, chemical etching, electrode bodning, etc., can move into and in such film to undesirably change the electric parameters of the device.

In order to eliminate or reduce the effect of the alkali ions, the phosphorus glass film (hereinafter referred to as P O -SiO, glass film) is formed in superposed relation on the SiO, (T.O.) film, whereby the alkali ions are trapped by the P,O,-Si0 glass. In a process for producing the I,O -SiO glass film, a sample consisting of a silicon substrate and the SiO, (T.O.) film is placed into a gaseous atmosphere containing phosphorous oxychloride POCl nitrogen and oxygen gases kept at a temperature at which the POCl, decomposes to deposit as a compound P,O on the SiO, (T.O.) film. The deposited P,O, and SiO, (T.O.) are heated to mutually diffuse, whereby a glassy mixture is formed having a thickness and a P 0, concentration which are determined in accordance with the heating temperature and heating time.

Such dual films are necessary for passivating the device, provided that the inclusion of the alkali ions can not completely be avoided. However, it is impossible to completely remove the alkali ions from a practical point of view.

Although the undesirable influence of alkali ions can be avoided by employing the above mentioned dual film,it is a severe drawback that the P O -SiO glass film is very poor in moisture resistance.

Therefore, it is required to protect the dual film from the moisture by means of another film of good moisture 5 resistance. In order to solve the above problem, there have been provided semiconductor devices having a film with three layers or a triple film. A silicon nitride film (hereinafter referred to as Si N film) is one of typical films with good moisture resistance. Since the Si N film has not only a good resistance to moisture, but also a considerably excellent resistance to alkali ions, this is a very suitable material as a passivating film.

According to a process for producing the Si -,N film, a silicon substrate is placed into a gaseous atmosphere containing monosilane SiH, and ammonia NI-I kept at a temperature at which the compounds react to form silicon nitride Si N as a deposit.

After many attempts in employing the Si N film superposed on the dual film mentioned above, it has been experienced that cracks occur almost every time in the Si N filni or in the three-layer film. Although the inventors have notyet ascertained frully what cause generates the cracks, the results of the inventors experiments indicate that one of these causes may be a big difference in thermal expansion coefficients between the Si N, and P,O,,-Si0 glass films, and that another may be a chemical reaction which takes place between the Si N and P 0 The inventors have made many at tempts with respect to choice of film thickness, filmforming condition, heat-treatment or aging for the film and so on in order to solve the above problem; however, these attempts did not succeed.

Although it may be possible to slightly reduce the tendency of crack occurrence by reducing the thickness of the Si N film, a number of pin-holes are then left in the film thereby deteriorating the moisture resistance and resistance to chemicals. As a result, the semiconductor device tends to become influenced by environmental conditions. Therefore, there exists a minimum thickness of the Si N film required for attaining the desired passivating effect. The inventors have concluded in accordance with the results of their experiments that the minimum thickness of the SI3N4 film must be at least 800 angstroms, especially 1,000 angstroms. In case of such an Si N, film, however, the inventors experiments have proved. that the crack occurrence of the Si N, film formed by conventional methods can scarcely be avoided. This. may be a reason why the sl b], film has not been used as a passivating film on an industrial scale.

Accordingly, it is an object of the present invention to provide a semiconductor device having an improved composite passivating film.

It is another object of the present invention to provide a semiconductor device having a four-layer or quadruple passivating film, each layer being arranged and superposed in such order that no crack occurs in the film, whereby semiconductor devices excellent in 0 rate.

According to the present invention there is provided a semiconductor device having such a four-layer passivating film which comprises a first silicon dioxide film body, a phosphorous glass film superposed on the first silicon dioxide film, a second silicon dioxide film formed by chemical vapor deposition on the phosphomoisture resistance can be produced with a high yield formed by thermal oxidation on the surface of a silicon rous glass film, and a silicon nitride film formed on the second silicon dioxide film.

The above objects and other objects and features of the present invention will be apparent from the following detailed description, taken in conjunction with the attached drawings in which:

FIG. 1 is a graph showing the number of pin-holes in an Si N film;

FIG. 2 is a graph showing a curve of thickness limit with respect to crack occurrence in P O -SiO glass and Si N film;

FIG. 3 is a graph showing relationships between thickness limit with respect to crack occurrence and the temperature at which Si N is deposited on an SiO film formed by thermal oxidation;

FIG. 4 is a graph showing thickness limit with respect to the crack occurrence in an SiO film formed by chemical vapor deposition and an Si N film;

FIG. 5 shows the radius of curvature of semiconductor bodies having various passivating films and that of a silicon body as a comparison;

FIG. 6 is a graph showing the relationship between the concentration of P in a P O -SiO glass film and the thickness limit with respect to crack occurrence in an Si N film, and

FIGS. 7a through 7e explain a process for manufacturing a semiconductor device according to the present invention.

According to the experiments conducted by the inventors of the present invention, it has been found that an Si N film should have at least about 1,000 angstroms in thickness in order for sufficiently reducing the pin-holes. This can be understood from the facts shown in FIG. 1, in which the Si N film was formed under the most proper and favorable condition, i.e., in which the flow rate of ammonia gas NH to monosilane SiI-I is 25 by volume and the temperature for depositing Si N is 830 C. Although the pin-holes present in the Si N film can be reduced by changing the forming condition, other disadvantages may take place by such a change. Therefore, the minimum thickness of the Si N film is about 1,000 angstroms, in the present invention. The upper limit of thickness of the Si N film will be explained later on in accordance with other information derived from the present invention.

As stated already, in the semiconductor devices having a composite passivating film of the Si N and P O -SiO glass films, it was almost impossible to avoid the crack occurrence. In order to analyze the cause of crack occurrence, the inventors investigated the thermal expansion coefficients with respect to silicon, P O -SiO, glass, SiO (T.O.), SiO (CVD) and Si N, Since the thermal expansion coefficients of P O -SiO glass and SiO, (CVD) films could not be determined clearly, they were estimated in accordance with the degrees of the radius of curvature, i.e., the degrees of bend of the samples; namely, one of which comprises a silicon body of 50 microns in thickness and a l,O,,SiO glass film of 1,500 angstroms in thickness, and another comprises the same substrate as mentioned above and an SiO, (CVD) film of 1,000 angstroms in thickness. The silicon body and the film materials are arranged in accordance with the thermal expansion coefi'icients thereof as follows:

SiO, (T.O.) P O -SiO glass Si SiO (CVD) Si N The thermal expansion coefficients of the above materials are shown in the following Table 1':

TABLE 1 Material thermal expansion coefficient SiO,(T.O.) 4.0 X l0 /C P,O Si0, glass of the order of l0"/C SiO, (CVD) of the order of l0 /C Si N, 3.85 X |0'/C Si 3.74 X l0 /C It will be appreciated from the above facts that a big difference in thermal expansion coefficients exists between the P O -SiO glass and Si N films and that such big difference will cause the cracks in the films.

According to FIG. 2 which shows a curve of limit in thickness with respect to crack occurrence, it will be appreciated that an increase in the thickness of P O -SiO glass film has a tendency to increase crack occurrence in the Si N film. As stated already, the minimum thickness of Si N film is about 1,000 angstroms. From this point of view (pin-holes), the larger the thickness of the Si N film, the more proper the film will become; however, as understood from the results shown in FIG. 2, there exists an upper limit of the thickness of Si N film. In connection with this matter, the inventors concluded a thickness of Si N of about 1,000 to about 3,000 angstroms is proper.

On the other hand, the thickness of P O -SiO glass film necessarily has to be limited within a particular range from the view point of the alkali trapping effect and soundness thereof.

The inventors believe that the P O -SiO film should have at least about 500 angstroms in thickness in order to expect a sufiicient alkali trapping effect. Since the P O -SiO film is formed by the mutual diffusion of P 0 and SiO which is carried out by heating, and since the thickness of the P O -SiO glass film is determined in accordance with the temperature and the time of heating same, the dimensional change of a predetermined PN junction takes place when the body is excessively heated. Accordingly, the maximum thickness of P O -SiO glass film is about 2,000 angstroms when superposed with the Si N film.

It has been found that the crack occurrence is not necessarily caused only by the difference in the thermal expansion coefficients between the Si N and P O -SiO glass films. This will be understood from the results shown in FIG. 3. A curve I illustrates in this figure the limit in thickness concerning samples provided with an SiO (T.O.) film of 7,000 angstroms and a P O -SiO film of 1,400 angstroms in thickness, and the curve [I the limitin thickness concerning samples provided with an SiO (T.O.) film of 7,920 angstroms in thickness. All the Si N films were formed in accordance with the condition of a Sil-I flow amount of 5 cc/min., NH /SiH is 30 by volume and a nitrogen flow amount of 10 l/min. According to FIG. 3, it will be understood that the formation of the P,O=,-SiO film between the si N film and the SiO (T.O.) film will lower the thickness limit of the Si N film as shown by the curve I. From the fact that an increase in temperature for depositing Si N lowers the thickness limit, the crack occurrence may be caused by another cause. In connection therewith, the inventors have therefore thought the crack may also be caused by a chemical reaction which takes place between the Si N and PzO -siO glass films and that this disadvantage will be avoided by inserting a kind of filtering film between the above two films.

After the inventors investigations, it has been found that a silicon dioxide film formed by chemical vapor deposition, i.e., an SiO (CVD) film, is suitable as a film to be inserted between the Si N and P O -SiO films. The SiO (CVD) film has a thermal expansion coefficient adjacent to those of the Si N and P O -SiO films. Furthermore, this film has no reactivity with respect to Si N By the formation of the SiO, (CVD) film between the above two films, the crack occurrence will be avoided effectively. It goes without saying that the Si (CVD) film should have a proper thickness in order to attain its effect. According to applicants investigations, it was found an SiO (CVD) film of about 1,000 to about 6,000 angstroms in thickness is proper. When the SiO (CVD) film has a thickness smaller than 1,000 angstroms, the cracks in the Si N films can not be prevented sufficiently. In the case where the film is thicker than 6,000 angstroms, it takes too long a time to form such a thick SiO (CVD) film and the cracks tend to occur in accordance with the big difference in thermal expansion coefficients between the SiO (CVD) film and the Si l\l film. The reasons for the thickness of the SiO, (CVD) film will be understood from the results shown in FIG. 4 which shows the limit in thickness with respect to the crack occurrence between the SiO (CVD) film and the Si N film. From the results will be appreciated that with an increase in the thickness of SiO (CVD)film the tendency of crack occurrence will become small. In the experiments, an SiO (T.O.) film of 6,200 angstroms and a P O -SiO, glass (12 mol percent of P 0 film of 1,550 angstroms in thickness were previously formed under the SiO (CVD) film.

The investigations have also proved that the cracks occurring in the films are generated by the difference in the Youngs modulus between the film materials under consideration. The film materials and silicon body are arranged in accordance with the magnitudes of Youngs modulus as follows:

SiO (CVD) P O -SiO, glass SiO,(T.O.) Si Si N The Youngs moduli in concrete value are shown in The present inventors undertook tests of which the results are shown in FIG. 5. In FIG. 5, the ordinate represents the radius of curvature in millimeters, and the abscissa the kinds of samples. In samples designated by A, D represents the radius of curvature of silicon and (O) represents that of the sample comprising a silicon substrate and an Si N film of 3,940 angstroms in thickness. Similarly, in samples 8, (Q represents samples comprising a silicon substrate and an SiO '(T.0.) film of 7,450 angstroms, (x) samples comprising 'a siliconsubstrate and an SiO, (T.O.) film of 7,300 angstrorns, and (J) a sample comprising a silicon substrate,

an SiO (T.O.) film of 7450 angstroms and an Si N film of 3,900 angstroms; in samples C, represents samples comprising a silicon substrate, an SiO, (T.O.) film of 5,810 angstroms and a P O -SiO glass film of 1,700 angstroms, (x) samples comprising a silicon substrate, an SiO (T.O.) film of 5,450 angstroms and a P-zOs-SlOg glass film of 1,510 angstroms, and (O) samples comprising a silicon substrate and an Si;,l\l, film superposed on the P O -SiO glass film of 5,810 angstroms; and in samplesD, Q represents a sample comprising a silicon substrate and an SiO (CVD) film of 5,240 angstroms superposed on the P O -SiO glass film used as( samples in C, and (0) samples comprising a silicon substrate and an Sl b], film of 3,900 angstroms superposed on the above SiO (CVD) film. In this experiment, all the samples designated by (G), which have the Si N film, bend toward the direction opposite to that of samplesdesignated by Q and (x).

Since the SiO, (T.O.) and P O -SiO glass films have thermal expansion coefficients smaller than that of silicon, samples provided with these films bend toward the side of films unformed. In the case that an Si Nr film which has a thermal expansion coefficeint larger than that of silicon and has the largest Youngs modulus in the materials under discussion, is formed, however, the samples bend toward the side of films formed and the radius of curvature becomes larger as compared with the case of no Si N film, i.e., by theformation of the Si N film the degree of bend is made small.

It is a remarkable and unexpected effect for the purpose of preventing the crack occurrence, that in combining the SiO (CVD) film with the Si N film, the radius of curvature becomes smaller than in the case of no SiO (CVD) film.

Fromthe various points of view mentioned above, it

will be understood that the SiO (CVD) film is most proper for attaining the objects of the present invention. The present inventors have discovered that the tendency of crack occurrence can be lowered by reducing the concentration of P 0 in the P O -SiO glass film as is understood from the results shown in FIG. 6. Ingeneral, from the practical point of view, the P 0 concentration employed is about 12 to 13 mol percent; however, by lowering the P 0 in the P,0 -Si() glass film to about 9 mol percent, the tendency of the crack occurrence can be reduced remarkably. Since the P 0 concentration is determined by the heating temperature for mutually diffusing P 0 and SiO,, the lowest P 0 concentration is necessarily determined by the need to avoid the dimensional change of PN junction in the device. In connection therewith, the inventors therefore proposesuch a P 0 concentration as is in the range of about 5 to about 9 mol percent.

FIGS. through 7e explain one of the processes for manufacturing the semiconductor device according to the present invention. In this example, the process showing a particular technique with respect to forming the passivating films is employed for the purpose of preventing the crack occurrence. According to the investigation, the cracks arise in the parts where the con centric stress tends to occur, particular in edges of holes or grooves "of the films. In order to solve this problem, the process shown in FIGS. 7a through 7e is employed in such a manner that the Si N film is so formed as to avoid the crack occurrence.

First,-a silicon wafer having a PM junction formed between a p-type region 1 and an n-type region 2 is prepared. The n-type region 2 is formed by doping an ntype impurity through an opening 5 which is formed in an SiO (T.O.) film 3. During the impurity doping, a thin SiO (T.O.) film 4 is also formed on the surface exposed at the opening as shown in FIG. 7a.

After the desired PN junction is formed, a P O -SiO film 6 is formed on the SiO (T.O.) film by heating the device to diffuse P deposited in an atmosphere containing IOCI and N and O gases on the SiO (T.O.) film. In this step, the thickness of Si0 (T.O.) film becomes small. Preferably, the thickness of the SiO (T.O.) film is from about 4,000 to about 15,000 A. The composite thickness of the first SiO (T.O.) film and the phosphorous pentoxide film is about 6,000 to about 16,000 A. Then, the P O -SiO film is coated by an SiO (CVD) film 7. The composite thickness of the first SiO film and the phosphorous glass film and the second SiO film may range from about 7,000 to about 18,000 A. A part 8 corresponding to the opening is left as a concave depression as shown.

Next, the parts of the composite film corresponding to the opening and a groove for scribing the wafer are removed by a chemical etching method.

If an Si N is in contact with the silicon body and it is removed by etching the surface of the body becomes rough. In order to avoid this problem, thin SiO (T.O.) films 9 and 11 are formed on the exposed surfaces in opening and 12 as shown in FIG. 70, prior to forming the Si;,N film.

As shown in FIG. 7d, the surfaces of the SiO (CVD) film 7 superposed on the P O -SiO glass film, the thin SiO (T.O.) films 9 and 11 and edges of the films are covered with an Si N film 14 deposited from a gaseous atmosphere containing NI-I and SiH,. At the side part 17 of the Si N film 14 no concentric stress generates even if the parts 17 and 18 are removed by chemical etching as shown in FIG. 72. The opening is filled with a metallic electrode formed by vacuum evaporation. After the electrode 20 is formed, the wafer is scribed in accordance with the groove 19 to produce a desired semiconductor device having an improved composite passivating film.

While we have shown and described only one embodiment in accordance with the present invention, it is understood that the same is not limited thereto but is susceptible of numerous changes and modifications as known to those skilled in the art. For example, while the present invention has been described specifically with reference to a silicon wafer with a particular PN junction, the passivating film of the present invention is equally applicable to all other types of semiconductor devices requiring passivating films for similar reasons. Hence, we do not wish to be limited to the details 5 shown and described herein but intend to cover all such changes and modifications as are encompassed by the scope of the appended claims.

What we claim is:

1. In a semiconductor device comprising a silicon body having at least one edge of a PN junction extending to a plane surface of said silicon body, a passivating film covering said plane surface including said edge surface so that said passivating film protects said device from undesirable irreversible change in electric parameters, and an electrode member electrically connected to said plane surface through an opening formed in said passivating film,the improvement wherein said passivating film comprises:

a first silicon dioxide film covering said edge surface, formed by thermal oxidation of said silicon body and having a thickness of about 4,000 to 15,000 angstroms so that an increase in the surface change density at the edge surface is minimized;

a phosphorous glass film having a concentration of P 0 of about 5 to 9 mol percent for reducing the effect of alkali ions present in said first silicon dioxide film, formed on said first silicon dioxide film by thermal diffusion of phosphopentoxide and having a thickness of about 500 to 2,000 angstroms;

a silicon nitride film for protecting said phosphorous glass film from moisture and having a thickess of about 1,000 to 3,000 angstroms; and

a second silicon dioxide film for moderating the stress and preventing the chemical reaction occuring between said phosphorous glass film and silicon nitride film, interposed between said phosphorous glass film and said silicon nitride film and having a thickess of about 1,000 to 6,000 angstroms.

2. A semiconductor device according to claim 1,

wherein the thickness of the first silicon doixide film and the phosphorous film is about 6,000 to about 16,000 angstroms.

3. A semiconductor device according to claim 1, wherein the thickness of the first silicon dioxide film, the phosphorous glass film and the second silicon dioxide film is about 7,000 to about 18,000 angstroms.

* III 

2. A semiconductor device according to claim 1, wherein the thickness of the first silicon doixide film and the phosphorous film is about 6,000 to about 16,000 angstroms.
 3. A semiconductor device according to claim 1, wherein the thickness of the first silicon dioxide film, the phosphorous glass film and the second silicon dioxide film is about 7,000 to about 18,000 angstroms. 